1. Field of the Invention
Silicon carbide (SiC) is a well-recognized ceramic material with a wide variety of applications because of its low density, high strength, high thermal stability, and high resistance to oxidation and corrosion. These characteristics make SiC a suitable material for components in electronic devices and for potentially replacing metal in engine parts. Silicon carbide is also suitable for use in low friction bearings, thermal and environmental barrier coatings, and wear resistant components (e.g. brakes).
While it is desirable to replace existing materials with SiC in most industries, the hardness and non-melting characteristics of this ceramic material makes it difficult to process by conventional methods. One solution to this problem is to use polycarbosilances as precursors of SiC because of their solubility in organic solvents, moldablity, spinnablity, cross-linkablity and high yield on pyrolysis.
2. Background Art
The first commercial polycarbosilane which was used as a precursor of SiC was disclosed by Yajima et al. in U.S. Pat. No. 4,100,233. This precursor of SiC was prepared by coupling dimethyldichlorosilane with sodium in toluene, followed by a rearrangement reaction of poly(dimethylsilane) in an autoclave at high temperature. The resultant polymer has a major repeat unit, [SiMeHCH2] and can only exist as solid form due to some un-converted Si—Si bond. However, the use of sodium and high temperature treatment tend to incur high manufacturing cost which diminishes the viability of this preparation method.
A polycarbosilane disclosed by Yajima has been used to prepare Nicalon fiber via a melt spinning process. An oxygen curing process for retaining the shape of the Nicalon fiber occurs before pyrolysis. This oxygen curing step is necessary to supplement the lack of efficient cross-linkable functional groups in the Yajima polycarbosilane. However, excess oxygen in the SiC is created after pyrolysis. In addition to the excess oxygen, excess carbon is also generated after pyrolysis. The excess carbon is due to a carbon to silicon ratio of 2:1 in the polycarbosilane precursor. The residual oxygen and excess carbon have negative effects on the long-term stability of ceramic fibers like Nicalon fiber.
Another disclosure (U.S. Pat. No. 4,826,892) by Shimada et al. teaches a method of making a phenyl substituted polycarbosilane or polycarbosilastyrene, similar to the Yajima polymer. While the fiber made from this precursor contains less oxygen, the cost of manufacturing this polymer is comparable to Yajima's polymer.
Whitmarsh et al. (U.S. Pat. No. 5,153,295) disclosed a branched polycarbosilane, [SiH2CH2]n, prepared by Grignard reaction of chloromethyltrichlorosilane in ether, followed by reduction with lithium aluminum hydride (LiAlH4). While this polycarbosilane has a high SiC yield, due to a 1:1 silicon:carbon ratio, it can only exist as a liquid at room temperature due to its low glass transition temperature (TG).
U.S. Pat. No. 5,270,429 by Michalczyk disclosed an extensively branched chloropolycarbosilane having a formula: [CH(Cl)zSiMe(H)x]Hy. The extensive branching in this polycarbosilane is a steric hindrance for the preparation its copolymer with (dichloromethyl)methyldichlorosilane. This particular characteristic causes incomplete coupling of all chlorine atoms during a Grignard reaction of monomer, (dichloromethyl)methyldichlorosilane. Excess chlorine from incomplete coupling with magnesium (Mg) in the Grignard reaction is not desired in SiC. In comparison, the use of sodium (Na) in a corresponding Wurtz coupling reaction drives the coupling to completion leaving no uncoupled chlorine. However, the complete coupling of chlorine produces an insoluble solid as end polymer, [CHSiMe]n, with limited use because processing such an insoluble solid as a precursor is difficult and costly.
In view of the foregoing, a need exists to overcome one or more of the deficiencies in the related art.